FORGETTABLE FIRST: Jenrry Mejia delivers during his major league debut, a one-inning stint in which he allowed three hits and a run during the Mets' 7-6, 10-inning loss to the Marlins last night in Flushing. Photo: N.Y. Post: Charles Wenzelberg

Jenrry Mejia’s major league debut didn’t live up to expectations, but his rough performance was downplayed last night by both his manager and catcher.

“He does what he does. He throws strikes,” Barajas said afterwards. “They got some hits and that’s going to happen.”

That absolutely happened last night, as the Mets’ 20-year-old flame-throwing prodigy served up a run on three well-struck hits in pitching the sixth inning of a 7-6, 10-inning loss to the Marlins.

Mejia struck out one, reached 97 mph and retired Marlins star Hanley Ramirez to end the inning, but the righty, who never pitched above Double-A before, was hit hard immediately and didn’t fool many batters.

Manuel, though, praised Mejia’s strike-throwing ability (nine of 13 pitches for strikes) and said the rookie is “going to be a big help for us.”

He said he’d have to check the film to see if Mejia was getting the late life on his pitches that he’s known for. Both Mejia and Barajas, though, said the life was there — that may be good or bad, considering that he got hit.

“His movement was good. His velocity was good,” the catcher said. “I didn’t see any jitters. I didn’t see anything out of the norm.”

Mejia was the first one out of the pen last night, coming in for the sixth. It was seemingly a good spot to put the rookie in — the Mets were already trailing 4-1, so he didn’t have to protect a lead.

Mejia, who said he wasn’t nervous, started off with a 95-mph called strike to Cody Ross, but his second pitch was ripped to right-center for a single. His next pitch was then driven for a double to right by Gaby Sanchez, giving the Marlins second and third with none out.

Mejia, who admitted to being “a little bit surprised” that he got hit early, struck out pitcher Ricky Nolasco on three pitches, the last a 97-mph heater. But his first pitch to Chris Coghlan was lined up the middle for an RBI single before Cameron Maybin hit into a force play and Ramirez grounded to short.